Incorporation by Reference
The present application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 2004-0101267 filed on Dec. 3, 2004. The content of the application is incorporated herein by reference in its entirely.
1. Field of the Invention
The present invention is in the field of a circuit for compensating for an offset voltage of a monitoring photodiode. More particularly, the present invention relates to a circuit for compensating for an offset voltage by adding current sources and offset resistances according to an offset voltage measured in a photodiode test.
2. Description of the Related Art
Since the advent of CDPs, the optical recording industry has been greatly advanced, with the development of DVDPs, CDRWs, DVDRWs, COMBOs and SUPER COMBOs. Nowadays, HDTV broadcasting requires high-speed optical recording systems for recording and storing large data such as high quality images. In this regard, there is keen competition in the development of optical pickup devices, including CDRWs, DVDRWs and Bluray recorders.
To record data on CD or DVD media by use of high-speed laser signals, an optical recorder must be supported by a fast and accurate control of laser diode power to which a technology for laser power monitoring PDIC (hereinafter referred to as “MPD” (monitoring photodiode)) is essential. Particularly, an MPD must have small absolute values of offset voltages in order to accurately monitor laser power.
With reference to FIG. 1, a circuit diagram of a conventional MPD is shown. In the MPD, a photodiode 11 converts incident laser optical signals into current signals. These current signals are transformed into voltage by an I-V amplifier 12, followed by amplification of the voltage in a GAIN amplifier 13 before being output. The DC level of the output voltage should be identical to that of the standard voltage in the absence of input optical signals. When no input optical signals exist, the output voltage is expressed as VOFFSET. A high VOFFSET makes it difficult to accurately measure the quantity of optical signal incident on the photodiode 11. The VOFFSET is generated by error values of an OP AMP used in both the I-V amplifier 12 and the GAIN amplifier 13. The error values arise from the non-ideal characteristics of the OP-AMP. The elimination of such error values is conventionally achieved by providing a compensation current Ioffset—comp to ROFFSET to control the VOFFSET as seen in FIG. 2.
In FIG. 2, the following equation is obtained:VOUT=VX2+VIN—OFFSET  (1)
Detected inside of the GAIN amplifier, VIN—OFFSET is caused by the fact that the GAIN amplifier is not ideal.
Also, the following relationship is established in FIG. 2:VX2=VREF+VRoff  (2)
In the circuit of FIG. 2, the current inside the GAIN amplifier is ideally zero when VIN=0. Because the GAIN amplifier is not ideal, an input current is generated inside of the GAIN amplifier. Where the input current is expressed as Ib—diff, VRoff=Ib—diff×Roffset is obtained. The substitution of Equation (2) into Equation (1) results in:
                                                                        V                OUT                            =                                                V                  REF                                +                                  V                  Roff                                +                                  V                  IN_OFFSET                                                                                                        =                                                V                                      X                    ⁢                                                                                  ⁢                    1                                                  +                                  (                                                            I                      b_diff                                        ×                                          R                      offset                                                        )                                +                                  V                  IN_OFFSET                                                                                        (        3        )            
When the offset compensation current Ioffset—comp is provided as in FIG. 2, VOUT can be rewritten as follows:VOUT=VX1+[(Ib—diff+ioffset—comp)×Roffset]+VIN—OFFSET  (4)
If VOUT=VX1 as in the ideal case, the following relationship can be elicited from Equation (4)VIN—OFFSET=−(Ib—diff+Ioffset—comp)×Roffset  (5)
Where Equation (5) is satisfied, the following is obtained:VOUT≈VREF≈VX1  (6)
Therefore, the offset voltage can be compensated for by additionally installing a current source 16 for providing Ioffset—comp in the circuit as in FIG. 2 and by controlling Ioffset—comp so as to satisfy Equation 5.
Monitoring the very minute power of laser diodes, an MPD circuit must be able to precisely measure VOFFSET values and thereby accurately detect the quantity of optical signals. An error allowance is typically in the range of ±12 mV, but is required to be as low as ±3 mV in some recent applications.
As described in FIGS. 1 and 2, the offset compensation circuit based on the application of the offset compensation current source can improve the offset characteristics of an MPD but only to a very limited extent. In fact, because mass-produced amplifiers have varying offset values, the conventional offset compensation circuit suffers the disadvantage of having limited compensation ability.
In this regard, Japan Pat. Laid-Open Publication No. 2004-120311 suggests a solution to the problem of limited compensation ability, disclosing that a circuit comprising two preamplifiers and a differential amplifier for differentially amplifying the output voltages of the preamplifiers can not generate an output offset voltage in output voltage by selectively switching feedback resistors connected to the amplifiers.
However, this and other known offset compensation circuits are difficult to apply to the process of producing MPDs. Accordingly, there is a clear need for an offset compensation circuit that is suitable for the production of MPD.